Limb for breathing circuit

ABSTRACT

A limb for a breathing circuit manufactured from very thin walled polymer materials has an elongate axial reinforcing spine lying freely inside the conduit and fixed to each end connector. The spine is laterally compliant but axially stiff. The spine provides resistance to tensile and compressive loads on the conduit, including that induced by prevailing internal pressures.

BACKGROUND TO THE INVENTION

The present invention relates to components for breathing circuits andin particular to limbs for breathing circuits.

SUMMARY OF THE PRIOR ART

In assisted breathing, particularly in medical applications, gases aresupplied and returned through conduits. Such conduits are ideally lightand flexible to ensure the greatest level of comfort for the patient.

As taught in our prior patent application AU 43823/01 thin membranewalls are particularly used in breathable membrane applications wherethe passage of water vapour through the membrane but not the passage ofliquid water is desired.

Thin walled conduits may include helical or annular reinforcing ribswhich improve resistance to crushing and pinching, while still allowingthe conduit to be flexible in order to maintain patient comfort. Adisadvantage of these types of flexible conduits is their lack ofstiffness. The extremely thin walls of these types of conduits providevery little resistance to tensile, compressive or torsional forces.While annular or helical ribs, whether inside, outside or between layersof the conduit wall, do provide some longitudinal stiffness, theseconduits are still prone to large axial displacements both compressiveand tensile. This can lead to substantial internal volume changes underfluctuating breathing pressures, potentially significant enough todisrupt automated ventilation. Our prior art patent application taughtprovision of external longitudinal reinforcing in the form of a set ofaxial polymer threads bonded to the radial support bead. However thesehave the disadvantage of being easily caught or snagged.

A further disadvantage of very thin walled conduits is a reduceddurability of the very thin membrane making up the walls of the conduit.The very thin membrane may be more susceptible to piercing from sharpobjects and/or plastic deformation from tensile forces.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a limb for abreathing circuit, which will at least go some way towards improving onthe above or which will at least provide the public and the medicalprofession with a useful choice.

Throughout this specification the term very thin walled conduit means aconduit where under the intended prevailing conditions the conduit wouldbe subject to excessive axial compression, e.g. a conduit formedaccording to a method as described in U.S. Pat. No. 3,910,808 using aSYMPATEX film having a thickness less than 50 microns.

In one aspect the invention consists in a limb for a breathing circuitcomprising:

a very thin walled conduit having a first end and a second end and abreathing gases pathway therebetween,

a first connector fixed to said first end of said conduit,

a second connector fixed to said second end of said conduit, and

an elongate reinforcing member lying freely within said very thin walledconduit along a non-tortuous path from one end of said conduit to theother end of said conduit, and connected with said first connector andsaid second connector.

Preferably said connectors have a first end suitable for makingconnection with auxiliary equipment and a second end for makingconnection with a breathing conduit, and

an annular shoulder between said first end and said second end,

said second end extending along an axis and having a substantiallycircular cross section, and

said second end having at least one protrusion on an outer surface forinterlocking engagement with a helical rib of a breathing conduit.

In a further aspect the invention consists in a method for manufacturinga limb for a breathing circuit comprising:

providing a very thin walled breathing conduit having a first end and asecond end,

locating an elongate reinforcing member having a first and a second end,lying freely within said conduit along a non-tortuous path from one endof said conduit to the other end of said conduit,

fixing a first end connector with a first end of said breathing conduit,and a first end of said elongate reinforcing member, and

fixing a second end connector with said second end of said conduit andsaid second end of said elongate reinforcing member.

In a further aspect the invention may broadly be said to consist in alimb for a breathing circuit comprising:

a very thin walled conduit having a first end and a second end,

a first connector fixed to said first end of said conduit,

a second connector fixed to a second end of said conduit, and

a braided sheath surrounding said conduit and being fixed at and aroundone end to said first connector and at and around its other end to saidsecond connector.

In a further aspect the invention consists in a method for manufacturinga limb for a breathing circuit comprising:

providing a very thin walled breathing conduit having a first end and asecond end,

locating a reinforcing mesh having a first and a second end, over theoutside of said breathing conduit,

fixing a first end connector with a first end of said breathing conduit,and a first end of said reinforcing mesh, and

fixing a second end connector with said second end of said conduit andsaid second end of said reinforcing mesh.

To those skilled in the art to which the invention relates, many changesin construction and widely differing embodiments and applications of theinvention will suggest themselves without departing from the scope ofthe invention as defined in the appended claims. The disclosures and thedescriptions herein are purely illustrative and are not intended to bein any sense limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional side elevation of a single walled breathingconduit formed by applying a molten reinforcing bead on top ofoverlapping spirally wound thin film layers.

FIG. 2 is a cross sectional side elevation of a double walled breathingconduit formed in a manner analogous to the conduit shown in FIG. 1.

FIG. 3 is a plan view of a conduit forming device for forming theconduit depicted in FIG. 2.

FIG. 4 is a cross sectional side elevation of a single walled breathingconduit formed by applying a molten reinforcing bead so that it residesbetween the overlapping spirally wound thin film layers.

FIG. 5 is a plan view of a conduit forming device for forming theconduit depicted in FIG. 4.

FIG. 6 is an assembly perspective view of one end of a breathing limbaccording to a preferred embodiment of the present invention.

FIG. 7 is a partially assembled perspective view of the end of abreathing limb shown in FIG. 6.

FIG. 8 is a cross-sectional elevation of the ends of the breathing limbaccording to FIGS. 6 and 7.

FIG. 9 is an assembly perspective view of one end of a breathing limbaccording to a further preferred embodiment of the present invention.

FIG. 10 is a cross-sectional elevation of a breathing limb according toa further preferred embodiment of the present invention.

FIG. 11 is a partially assembled perspective view of one end of abreathing limb according to a further aspect of the present inventionincluding an outer reinforcing mesh.

FIG. 12 is cutaway view of the breathing limb of FIG. 11 showing theouter reinforcing mesh fixed at and around the end connectors.

DETAILED DESCRIPTION

The present invention relates to breathing conduits in general and inparticular to methods of providing reinforcement for very thin walledconduits used to provide a closed pathway for delivering gases to apatient. Consequently the present invention finds application inbreathing conduits fabricated from a variety of different materials andmanufactured by a variety of different methods. The conduits may besingle or multiple walled and may include breathable walls or portionsof breathable wall.

As a corollary of material cost and/or breathability of the material itis preferred that the conduit wall be manufactured to have a very thinwall, so much so that the conduit wall membrane may be insufficientlysturdy to be self supporting. Spiral or helical or annular reinforcingmembers may be provided on the tubular membrane to provide supportagainst crushing and pinching. The helical, spiral or annular supportingmembers may for example be formed from polymer plastic materials, suchas the material used in the wall of the conduit or having the same basepolymer. It has been found that breathing conduits such as thosedescribed above are extremely light, flexible and provide good crushresistance, however the conduits may also have reduced resistance toaxial deformation. Due to the very thin polymer film forming the wallsof the conduit, the resulting breathing circuit limb may have reducedaxial stiffness and may be prone to expansion, and contraction along theaxis of the conduit, due to axial or torsional forces. In use, axialforces arising from patient breathing may produce expansion and/orcontraction along the length of the limb. In one aspect the presentinvention provides a breathing circuit limb with improved axialstiffness. In a further aspect the present invention provides abreathing circuit limb with improved torsional stiffness.

Very thin walled breathing conduits such as those described above can befabricated by a number of different methods. The following describesseveral very thin walled conduits and associated methods ofmanufacturing very thin walled conduits to which the present inventionmay be applied.

Referring to FIG. 1 a cross section of the wall of a breathing circuitlimb is shown in which the flexible wall of the conduit is formed from avery thin film plastic membrane, and wound helically with edges ofadjacent turns welded together by a reinforcing bead. Supplied as tape,either pre-formed or extruded online, the very thin film 40 is woundhelically onto a former with adjacent edges 41 and 42 of tapeoverlapping. A helical supporting rib 43, provided in a molten state isthen laid on top of the overlap between adjacent turns. The helicalsupporting rib thermally and mechanically bonds the two adjacent stripswith the rib forming a flexible resilient conduit once cooled. Theresulting product is a single walled breathing conduit which is lightand flexible. Further embodiments of conduits formed by such a process,such as multiple walled conduits, can be formed by adding further stagesto the above described forming process.

Referring to FIG. 2 a double walled conduit may be formed by adding anadditional thin film layer 44 and supporting rib 45.

An example of forming apparatus suitable for manufacturing the doublewalled breathing tube product according to the embodiment described inFIG. 2 is shown in FIG. 3. The apparatus includes a former 1 preferablyof a known type including a plurality of rotating rods arranged around acentral support rod. The rods extend from and are rotated by a gearboxwithin a machine stock 2. At least in the tube forming region therotating rods follow a helical path. The pitch angle of the rodsrelative to the support rod controls the pitch angle of the tube beingformed. An example of such a machine is a spiral pipeline mandrelavailable from OLMAS SRL of Italy. Tube being formed on the former isrotated and advanced in the direction of arrow 3 by the movement of therotating rods. The advance speed of the former is selected relative tothe rotational speed so that the pitch of the helical laying of thestrip or tape on to the former 1 is a little less than the width of thestrip so that adjacent turns narrowly overlap. A first extruder 4extrudes a very thin tape 5 of breathable polymer materials. The tape 5deposits on the former 1 in a helical fashion by action of the former.The pitch of the helical deposition of tape 5 is slightly less than thewidth of tape 5. The helical deposition of tape 5 forms the innerbreathable wall 6 of the conduit. A second extruder 7 extrudes a bead 8of polymer material. The bead 8 deposits on the former over the joint oroverlap between adjacent turns of tape 5 forming a raised bead 9 alongthis join and welding the overlapping turns of tape 5. A third extruder10 extrudes a second tape 11 of breathable polymer. The second tape 11of breathable polymer is deposited on the former 1 to span betweenadjacent turns of bead 8. Adjacent turns of tape 11 overlap, formingouter breathable sheath 12. A fourth extruder 13 extrudes a secondmolten polymer bead 14. The bead 14 is helically deposited along theoverlap between adjacent turns of the second tape 11 and welds theoverlapping turns of tape 11. In addition to the bonding of the filmoverlap by application of the molten bead other active fusing techniquesmay be applied.

The resulting product is a double walled reinforced breathing conduitwith a space between the inner and outer walls. The breathing conduit ofFIG. 2 is manufactured by a method analogous to the method employed tomanufacture the conduit of FIG. 1. The forming apparatus shown in FIG. 3is effectively made up of two identical stages arranged in series.

The first stage of the former shown in FIG. 3 consists of film extruder4 and bead extruder 7. Film 4 is wound around former 1 while extruder 7extrudes a molten bead on top of the overlapping layers of film 5,resulting in a conduit such as that shown in FIG. 1. The second stageconsists of film extruder 10 and bead extruder 13. This second stageeffectively repeats the first stage over top of the conduit formed bythe first stage and results in the double walled breathing conduit ofFIG. 2.

Referring to FIG. 4, a conduit is shown according to another preferredmethod of manufacture of single walled breathing conduits. This methodis particularly suited to very thin walled conduits and is the subjectof a co pending patent application. The very thin film is arranged in aspiral or helix such that the edge portions of adjacent layers overlapand form the wall of a tube. Interposed the overlapping edges ofadjacent winds of film is a bead of polymer material 47 bonded with theoverlapping portions of film sealing the joint between windings andforming a continuous tube. The seam is formed between the edge of afirst layer of film 48 and the edge of a second, adjacent layer of film46 which is laid over top of the polymer bead while the bead is molten.The overlapping layer of film because it is so thin, follows the contourof the bead very closely and results in a smooth inner conduit wall.

An example of forming apparatus suitable for manufacturing the breathingtube according to an embodiment of the present invention described inFIG. 4 is shown in FIG. 5. The apparatus includes a former 15 includinga plurality of rotating rods arranged around a central support rod. Therods extend from and are rotated by a gearbox within a machine stock 16.At least in the tube forming region the rotating rods follow a helicalpath. The pitch angle of the rods relative to the support rod controlsthe pitch angle of the tube being formed. An example of such a machineis a spiral pipeline mandrel available from OLMAS SRL of Italy.

Tube being formed on the former is rotated and advanced in the directionof arrow 17 by the movement of the rotating rods. The advance speed ofthe former is selected relative to the rotational speed so that thepitch of the helical laying of the strip or tape on to the former 15 isa little less than the width of the strip so that adjacent turnsnarrowly overlap. A first extruder 18 extrudes a tape 19 of very thinfilm polymer materials. The tape 19 deposits on the former 15 in ahelical fashion by action of the former. The pitch of the helicaldisposition of tape 19 is slightly less than the width of tape 19. Thehelical deposition of tape 19 forms the wall 20 of the conduit. A secondextruder 21 extrudes a bead 22 of polymer material. The molten bead 22deposits between the overlapping portions of adjacent winds of tape 19and is sufficiently heated to weld to the strips of tape 19. Applyingthe molten bead between the overlapping layers of tape may improve theweld quality as both layers of tape that are to be welded are inphysical contact with the molten bead. The quality of the surface finishfor the inner surface of a breathing conduit is important, as a roughinner surface may hinder gases flow and contribute to more condensationto building up in the conduit. The above described constructiontechnique is especially suited to conduits fabricated from very thinfilm. The thin film is able to conform to the shape of the raised rib ofthe applied molten bead 22 during fabrication. By lapping very closelyonto the bead and wrapping around the bead, the very thin film maintainsa smooth inner surface on the finished conduit product as shown in FIG.4.

In addition to the bonding of the film to the molten bead betweenadjacent over lapping layers, other active fusing techniques may beapplied. Active methods may include hot air welding, hot rollers orradio frequency welding.

It will be appreciated that the above described breathing conduits andmethods of manufacture are provided as examples of the type of very thinwalled conduits to which the present invention may be applied. Theexamples have been chosen to illustrate the many possible variations andare not meant to be in any way limiting. Many further variations willpresent themselves to those skilled in the art. While some embodimentsof the present invention have been described as preferred and conveyparticular advantages over other embodiments many other combinations mayprove commercially useful.

Such variations may include:

-   (a) the utilisation of breathable material for the conduit walls or    parts of the walls;-   (b) single walled or multiple walled conduits, with or without space    between the walls may be formed by adding extra stages to the    forming process;-   (c) single layer or multiple layer walls;-   (d) very thin tape may be extruded at the time of forming, or    pre-formed and supplied to former on reels;-   (e) very thin tape may be provided as a laminate having a very thin    film layer and a reinforcing layer which is also permeable to water    vapour;-   (f) forming process may include a secondary thermal welding process;-   (g) molten bead may interpose layers or be applied on top of two or    more layers;-   (h) direct extrusion or drawing or blowing of a conduit;-   (i) forming a conduit from a very thin film with a longitudinal    seam;-   (j) providing a series of annular radial support beads rather than a    helical radial support bead.

The present invention may be broadly described as relating to methods ofreinforcing breathing circuit limbs so as to provide increased axial ortorsional stiffness, or both. While the present invention isparticularly suited to conduits having very thin walls, it will bereadily appreciated that application may also be found in moretraditional conduits if further reinforcement is desirable. The firstpreferred embodiment of the present invention describes the provision ofan axial spine and end connector whose primary function is to improvethe axial stiffness of a breathing circuit limb. The second preferredembodiment of the present invention describes an external reinforcingsheath or mesh and an end connector for use with such reinforcing in abreathing circuit limb. The reinforcing mesh is bonded to the limb atonly the ends of the limb where the conduit wall inserts into the endconnector. It will be appreciated from the following description thatthe end connectors described are suitable for use with either one, orboth, of the preferred embodiments of the present invention. While eachembodiment of the present invention is discussed in turn, it is in nosense meant to be limiting as the preferred embodiments may be employedseparately or together.

A first preferred embodiment of a breathing limb according the presentinvention will be described in detail with reference to FIGS. 6 to 8.The breathing limb has a conduit end connector 23 (or 49), suitable forconnecting a breathing conduit with a device, for example a gaseshumidification device or ventilator or mask. A first end of endconnector 23 is configured to mate with auxiliary equipment such as aventilator or mask, while the second end is configured to extend into abreathing conduit. The end view cross section of each end portion of theconnector is substantially circular. Between the two ends of the endconnector 23 is a shoulder region which makes the transition between therespective diameters of the connector ends. Preferably the shoulderportion has an annular recess 32, for receiving a securing collar orretaining sleeve 29.

The limb includes an elongate reinforcing member or spine 24 lyingfreely within conduit 25. Conduit 25 for example, is such as thosedescribed above. The second end of conduit end connector 23 has a recess26 adapted to receive an elongate reinforcing spine or rod 24. The spine24, runs the length of the conduit from the connector 23 at one end ofthe tube, down the inside of the conduit, and is secured in another endconnector 49 at the other end of the conduit. Preferably the spine issubstantially the same length as the conduit and follows a non-tortuouspath between the connectors. Because the spine (between the connectors)is preferably slightly longer than the conduit, it will not follow alinear path, but rather will bend into a shallow wavy and/or spiralform. It will also be appreciated that a spine slightly shorter than theconduit will also result in a degree of axial reinforcement. Whenassembled as described the combination of end connector and spine willprovide the breathing conduit with additional axial stiffness, bypotentially taking some of the axial forces and will therefore go someway to overcoming the above described disadvantages that arise from theuse of breathing conduits having extremely thin film walls. In thisembodiment it is preferable to choose the reinforcing spine (material,gauge and number) to be sufficiently stiff to resist buckling under thetransiently reduced internal pressures that could be expected duringpatient breathing and sufficiently stiff to provide improved axialstiffness to the conduit. Preferably the elongate reinforcing member ismanufactured from high density polyethylene having a Young's modulus(E), of approximately 0.88 GPa. Preferably the elongate reinforcingmember has a cross sectional are between 3 mm² and 12.5 mm². Preferablythe elongate reinforcing member has a minimum bending stiffness(EI=Young's Modulus*Second Moment of Area) for its cross section between693 N·mm² and 11,096 N·mm².

Although embodiments containing only one elongate reinforcing spine areshown, it will be appreciated by those skilled in the art that the endconnectors described could easily be modified to accommodate multiplereinforcing spines. In such multi-spine embodiments, care needs to betaken to ensure that the gases flow is not disrupted too detrimentally.A further important consideration when choosing the material, gauge andnumber of reinforcing members is to ensure that the breathing circuitlimb remains laterally flexible and thus maintain patient comfort.

The reinforcing spine is preferably made from a suitable approvedplastic material, such as high density polyethylene, or the samematerial as the end connectors if welding of the spine and endconnectors is selected for manufacture. In the preferred embodiment thereinforcing spine has a circular cross section to minimise any potentialstress raisers. The spine may be made from a variety of materials, andmay have a variety of cross sections being either solid or hollowwithout departing from the spirit of the present invention. Preferablyin hollow spine embodiments the spine is blind terminated at each end bythe end connectors. If the spine is hollow and has a narrow bore, thesize of the bore will be insufficient for general gases flow or gasesdelivery. The cross sectional area of the spine (measured from the outerperimeter of the cross section of the spine) is preferably less than 10%of the cross sectional area of the bore of the conduit so that gasesflow is not significantly disrupted. While the spine diameter is notlarge enough to facilitate significant gases flow (to a patient forexample) it may be used for other purposes such as pressure measurement,or pressure feedback. The spine may also include a heater element suchas a PTC (Positive Temperature Coefficient) heater or a resistanceheating element.

It is envisaged that there are several possible variants which may beemployed to secure the reinforcing spine and/or reinforcing mesh intoeach of the end connectors of the breathing circuit limb. The generalrequirements for the end connectors are as follows. The end connectorsmust provide a means for securely fastening the spine and/or reinforcingmesh so as to prevent pull out during use. Preferably the end connectorsare constructed such that assembly of the components during manufacturecan be achieved easily. A further consideration is that the endconnector when fastened to a breathing conduit to form the finishedproduct should be neat, tidy and preferably appealing to the eye of anend user. The following describes two alternative preferred embodimentsof the present invention which attempt to satisfy the abovementioneddesign objectives. It will be appreciated that the portion of the endconnector described which connects to equipment such as a ventilator ormask may be male, female or an androgynous type connector withoutdeparting from the present invention. Further, each end of a conduit mayhave the same or a different type of connector according to what type ofconnection is required. If a heater wire is included in the breathingcircuit limb (whether associated with the reinforcing spine or not) theend connector at at least one end will preferably be adapted to make anelectrical connection together with the gases pathway connection.

Referring to FIGS. 6 to 8, a connector according to a preferredembodiment of the present invention is shown. In order to provide astrong bond between the conduit and the connector, a portion of theconnector which receives the conduit is provided with outer raisedprotrusions 28 to cooperate with the helical reinforcing bead of theconduit. The protrusions 28 are arranged to cooperate with the pitch ofthe conduits helical reinforcing bead and preferably take the form of acontinuous thread. It will however be appreciated that the protrusionsmay be any number of discrete bumps arranged to cooperate with theconduit reinforcing bead. The raised thread 28 takes up a positionbetween the adjacent turns of the helical reinforcing bead 35 of theconduit. The thin wall of the conduit between the reinforcing bead isable to deform if necessary to accommodate the raised external thread ofthe end connector locking the components together. These featuresprovide a mechanical connection and resistance to the conduit beingpulled from the connector. As shown in FIG. 6 the portion of theconnector which receives the conduit is also provided with a recess orgroove 26 for receiving the reinforcing spine 24. Preferably the recess26 is substantially parallel with the extrusion axis of the connector.For assembly, the recess 26 provides a locating means for thereinforcing spine allowing the conduit to be threaded over the externalraised thread on the receiving portion of the end connector. Thereinforcing spine runs up the inside of the conduit and is received intorecess 26 of the end connector. The spine then emerges from the recess26 where an end portion 36 of the spine 24 is folded back on itselfaround the outside of the conduit wall. This feature provides amechanical interlocking of the spine around the conduit wall as well asproviding an end section of the spine that is in a position to beadhesively secured to the outer surface of the conduit wall.

In one preferred embodiment, illustrated in FIG. 6, a retaining sleeveor securing collar 29 is fitted over the assembled components. Thesecuring collar 29, is substantially cylindrical about an extrusionaxis. The retaining sleeve may include a raised portion 30 which resultsin a recess on the inside of the securing collar as shown in FIGS. 6 to8 for receiving the end portion of the spine 24 which is folded back onitself on the outside of the breathing conduit. Alternatively a recessmay be formed on the inner wall of the securing collar 29, without thepresence of an external protrusion. Preferably the recess issubstantially parallel with the extrusion axis of the securing collar.Alternatively, referring to FIG. 9 the end portion of the spine 36 maybe folded so it lies between the helical reinforcing bead 35 of theconduit and the raised thread 28 of the end connector 23.

The assembly is secured via a tubular retaining or securing collarsleeve 31. The retaining sleeve 31 and end connector 23 may be providedwith a positive initial location via a snap fit interaction between asnap fit portion 32 of the end connector 23 and the lip of retainingsleeve 31. Referring to FIGS. 6 to 9, a suitable adhesive such as EVA(Ethylene-Vinyl Acetate) glue can then be injected into the annularspace 33 formed between the receiving portion of the end connector andthe retaining sleeve. One or more small openings may be provided in thesecuring collar for the purpose of injecting glue into the annularcavity 33. The injected adhesive performs two functions, firstly theadhesive forms a seal between the conduit and the end connector.Secondly, the adhesive forms both an adhesive bond and a mechanical bondanchoring the conduit and spine to the end connector. The mechanicalbond is formed between the raised external threads of the end connectorand the cured glue which fills the annular space between the endconnector and the retaining sleeve. The mechanical bond between theraised threaded portion of the end connector and the breathing conduitis an important feature because there may be no adhesive between thesetwo surfaces. The cured glue must be hard enough to prevent the thinwalled conduit and reinforcing bead from deforming far enough to allowthe conduit to be pulled over the raised external thread.

An alternative preferred embodiment of an end connector will bedescribed with reference to FIG. 10. An end connector as describedpreviously with an external raised thread 28 on a conduit receivingportion of the connector is provided. In a similar manner to thatdescribed above the end connector is also provided with a recess 26 forreceiving a reinforcing spine. During assembly the reinforcing spine islocated in the recess before the helically ribbed breathing conduit isthreaded over the reinforcing spine and receiving portion of the endconnector. As described above, an end portion of the reinforcing spine36 is folded over the outside of the breathing conduit wall inpreparation for adhesive securing. Alternatively, end protion 36 may bepositioned as shown in FIG. 9. The assembly is then inserted into aninjection mould cavity so that a collar 38 (shown hatched) isovermoulded to perform the functions of securing and sealing asdescribed above.

Due to the axial compliance of very thin walled conduits, the length ofspine will contribute to the determination of the length of the limb. Inthe preferred embodiment the spine length is chosen such that whenfitted inside the conduit and secured to the respective end connectors,the conduit is elongated such that the conduit length is close to itsmaximum length (preferably within the elastic limit of the conduitwalls). In such a condition the wrinkling of the conduit wall isreduced, improving the performance of the breathing circuit limb withoutputting undue stress on the conduit wall due to axial tension generatedby the spine. The axial stiffness of the conduit is improved while limbflexibility is not significantly impaired. For this condition, the spineis preferably between 100.5% and 105% of the length of the conduit.

A second preferred embodiment of the present invention will now bedescribed in detail with reference to FIGS. 11 and 12. FIG. 11 disclosesa breathing circuit limb including an outer reinforcing sheath 27covering the entire length of the breathing conduit.

The reinforcing sheath 27 is preferably a braided mesh surrounding thebreathing circuit limb and is bonded to the limb only at the ends wherethe breathing conduit is inserted into the end connectors. All styles ofbreathing circuit limb end connector described above are suitable forreceiving and securing a reinforcing mesh according to the secondembodiment of the present invention. In each case the reinforcing sheathis located outside the breathing conduit wall and is secured at andaround the end connector at the same time as the conduit wall issecured. FIG. 11 shows an end connector having a breathing conduitreceiving portion which includes a raised external thread forcooperation with the helical reinforcing bead of the conduit. The endconnector may also include a recess or groove for receiving areinforcing spine as described in the first preferred embodiment of thepresent invention. During assembly the thin walled breathing conduit isthreaded over the end connector conduit receiving portion via theinteraction between the breathing conduits helical reinforcing bead andthe end connectors raised external thread. A tubular braided reinforcingmesh 27 is then installed over top of the breathing conduit. FIG. 11shows a reinforcing mesh 27 over a portion of breathing conduit. In FIG.11, the end portion of the mesh is not yet pulled all the way over theconduit ready for securing via retaining collar 29.

As previously described in the first preferred embodiment of the presentinvention two preferred methods of securing the breathing circuit limbcomponents are disclosed. The first method employs a securing collarpositioned over the breathing conduit and the conduit receiving portionof the end connector, forming an annular space which is then filled witha suitable adhesive such as EVA glue. The alternative securing methoddescribed in the first preferred embodiment of the present invention maybe adapted to secure the braided reinforcing sheath into the endconnector. In this overmoulded alternative the assembled components areinserted into an injection mould cavity so that a collar may beovermoulded to perform the functions of securing and sealing thecomponents of the breathing circuit limb. In this method the retainingsleeve is substituted for the overmoulded resin.

The braided reinforcing mesh may be applied to a breathing conduit as anonline process where the braid is formed at the same time as the conduitis formed, or alternatively a prebraided tube may be applied to abreathing conduit in a separate process. The braided mesh may befabricated from a variety of materials but is preferably polyethyleneterephthalate monofilaments.

In use the braided sheath contributes significantly to the tensile andtorsional stiffness of the breathing circuit limb. While there is nobonding between the reinforcing mesh and the breathing circuit limbalong the length of the conduit, it has been found that the braidedreinforcing mesh significantly improves torsional rigidity of thebreathing circuit limb. In this embodiment it is preferable to choosethe material, number, weave pitch and gauge of the braided filaments toimprove the conduits stiffness. When the limb is loaded in tension, thestretching of the reinforcing mesh causes the mesh tube to constrictradially. This radial constriction is resisted by the helicalreinforcing bead of the breathing conduit resulting in a strain limitingeffect for the breathing circuit limb. This effect significantlyimproves the breathing circuit limb strength and stiffness against axialtensile forces. The outer mesh sheath also provides an additionaladvantage by reducing direct contact between the user/environment andthe outer surface of the breathing conduit tube, therefore reducing therisk of puncture and damage. This feature significantly improves thedurability of the breathing circuit limb, and is especially suitable forconduits with very thin walls, such as those which may be found inbreathable walled limbs.

1. A limb for a breathing circuit comprising: a flexible conduit havinga first end, a second end and a breathing gases pathway therebetweendefined by wall, said wall comprising a very thin membrane having athickness less than 50 microns, and said membrane is insufficientlysturdy to be self supporting, said conduit is insufficiently sturdy suchthat said conduit would experience substantial internal volume changesas a result of said wall deforming due to fluctuating pressure withinsaid conduit from patient breathing; a first connector fixed to saidfirst end of said conduit; a second connector fixed to said second endof said conduit; and an elongate reinforcing member lying freely withinsaid conduit along a non-tortuous path from one end of said conduit tothe other end of said conduit, and connected with said first connectorand said second connector, such that said elongate reinforcing memberreinforces said conduit against said substantial volume changes thatwould occur in said conduit, but for said elongate reinforcing member,and said elongate reinforcing member does not include a passageway largeenough for gases delivery to a patient.
 2. A limb for a breathingcircuit as claimed in claim 1, wherein said elongate reinforcing memberhas a cross sectional area, measured from the outer perimeter, less than10% of the cross sectional area of the bore of said breathing conduit.3. A limb for a breathing circuit as claimed in claim 2, wherein saidelongate reinforcing member has a solid, substantially circular crosssection, and two ends.
 4. A limb for a breathing circuit as claimed inclaim 2, wherein said elongate reinforcing member includes a positivetemperature coefficient heating element.
 5. A limb for a breathingcircuit as claimed in claim 2, wherein said elongate reinforcing memberincludes a resistance heating element.
 6. A limb for a breathing circuitas claimed in claim 2, wherein the length of said elongate reinforcingmember is between 100.5% and 105% of the length of said conduit.
 7. Alimb for a breathing circuit as claimed in claim 1, wherein saidelongate reinforcing member is hollow, and said hollow elongatereinforcing member is blind terminated at each end by said firstconnector and said second connector.
 8. A limb for a breathing circuitas claimed in claim 7, wherein said hollow elongate reinforcing memberis hollow and said hollow elongate reinforcing member is large enough tobe used as a pressure measurement or feedback conduit.
 9. A limb for abreathing circuit as claimed in claim 1, wherein said connectors have afirst end suitable for making connection with auxiliary equipment and asecond end for making connection with a breathing conduit, and anannular shoulder between said first end and said second end, said secondend extending along an axis and having a substantially circular crosssection, and said second end having at least one protrusion on an outersurface for interlocking engagement with a helical rib of a breathingconduit.
 10. A limb for a breathing circuit as claimed in claim 9,wherein said protrusion is an external thread having a pitch suitablefor engagement with said helical rib of a breathing conduit.
 11. A limbfor a breathing circuit as claimed in claim 9, wherein said second endof said end connector has a recess substantially parallel with said axisfor receiving said elongate reinforcing member.
 12. A limb for abreathing circuit as claimed in claim 9, wherein said shoulder portionhas an annular recess for receiving a securing collar having anextrusion axis.
 13. A limb for a breathing circuit as claimed in claim12, wherein said securing collar has a recess substantially parallelwith said extrusion axis for receiving said elongate reinforcing member.14. A limb for a breathing circuit as claimed in claim 1, wherein saidelongate reinforcing member is resilient and does not plastically deformin use, under normal flexing and bending of said limb.
 15. A limb for abreathing circuit as claimed in claim 1, wherein said elongatereinforcing member has a cross sectional area between 3 mm² and 12.5mm².
 16. A limb for a breathing circuit as claimed in claim 1, whereinsaid elongate reinforcing member has a minimum bending stiffness between693 N·mm² and 11096 N·mm².
 17. A limb for a breathing circuit as claimedin claim 1, further comprising: a braided sheath surrounding saidconduit and being fixed at and around one end to said first connectorand at and around its other end to said second connector.
 18. A limb fora breathing circuit as claimed in claim 17, wherein said sheath is abraided tube braided from polyethylene terephthalate monofilaments. 19.A method for manufacturing a limb for a breathing circuit comprising:providing a flexible breathing conduit having a first end, a second endand a breathing gases pathway therebetween defined by a wall, said wallcomprising a very thin membrane having a thickness less than 50 micronssuch that said membrane is insufficiently sturdy to be self supporting,and said conduit is insufficiently sturdy such that said conduit wouldexperience substantial internal volume changes as a result of said walldeforming due to fluctuating pressure within said conduit from patientbreathing, locating an elongate reinforcing member having a first and asecond end, lying freely within said conduit, along a non-tortuous pathfrom one end of said conduit to the other end of said conduit, fixing afirst end connector with a first end of said breathing conduit, and afirst end of said elongate reinforcing member, and fixing a second endconnector with said second end of said conduit and said second end ofsaid elongate reinforcing member, such that said elongate reinforcingmember reinforces said conduit against said substantial volume changesthat would occur in said conduit, but for said elongate reinforcingmember, said elongate reinforcing member does not include a passagewaylarge enough for gases delivery to a patient.
 20. A method formanufacturing a limb for a breathing circuit as claimed in claim 19,further comprising: locating a reinforcing mesh having a first and asecond end, over the outside of said breathing conduit, fixing a firstend connector with a first end of said breathing conduit, and a firstend of said reinforcing mesh, and fixing a second end connector withsaid second end of said conduit and said second end of said reinforcingmesh.